This invention relates to apparatus for removing impurities from liquids by reverse osmosis, and relates more specifically to a tubular element and a method of mounting the element in a cartridge.
In reverse osmosis purification systems, a pressurized fluid containing one or more undesired components is applied to the surface of a semipermeable membrane which results in a solvent portion of the fluid passing through the semipermeable membrane and a preponderance of the undesired components becoming more concentrated at the membrane surface. Reverse osmosis systems have been used for many purposes, including the provision of relatively small amounts of purified water for home usage. These systems have frequently employed one or more suitably housed membrane elements which are continuously contacted with mineral-containing water. Purified water (which may be thought of as a solvent) slowly permeates through the membrane and is collected in a storage tank, such as a bladder tank, for use on demand.
A variety of different supports for the semi-permeable membrane and housings for the membrane elements have been used. Membrane elements frequently comprise a rigid tubular member around which is wrapped a spacing screen. A thin semipermeable membrane is then formed over the screen by casting or dipping the tubing into a solution of membrane-forming liquid, such as a cellulose acetate solution. Water passes through the cellulose acetate membrane and the screen, entering a center portion of the support tube, e.g., through perforations along the tube; pure water exits one end of the tube. Various methods and apparatus for applying semipermeable membrane layers to tubular units are known and form no part of the present invention. Examples of such methods are found in Thayer et al, U.S. Pat. Nos. 3,929,945, and 4,177,031. Similarly, any semipermeable membrane film may be used in the apparatus of the invention, including the cellulose acetate films disclosed in Loeb et al, U.S. Pat. No. 3,133,132, and Manjikian, U.S. Pat. No. 3,344,214. Brun et al, U.S. Pat. No. 3,834,546 discloses a flexible reverse osmosis element having a textile sheath support for a membrane, which may consist of a helical filament wound around a bundle of yarns.
This particular invention is concerned with a specific tubular semipermeable membrane element and methods of making and mounting the same. The element consists of an elongate rigid tube member having a plurality of longitudinal channels or grooves extending along the entire length of the tube. The tube has a monofilament having a diameter of about 5 mils helically wrapped with about 2-3 mil spacing around its exterior surface. A semipermeable membrane film is mounted on the monofilament surface by applying a liquid solution of membrane-forming materials thereto in any conventional manner, e.g. by extrusion, casting, or dipping, and permitting the solvent to evaporate to leave a thin, rigid, relatively uniform film. The membrane support is manufactured by cutting a plurality of V-shaped grooves into the tube, and wrapping monofilament around the tube, e.g., on a lathe.
In the past, membranes have been cast or otherwise fixed over a base generally comprising a fabric or fibrous material. Because the base material has a plurality of "ends" which may extend randomly through the membrane after casting, a relatively high failure rate caused by imperfections or "holidays" in the membrane surface has been realized. The monofilament base of the present invention has led to a failure rate of elements well below that heretofore realized.
The membrane elements of the invention are particularly adapted for use in relatively small cartridges which are suited for home use. The housing is designed to provide as much membrane surface area as is practical for each unit of volume of the housing; this permits a relatively small unit which may be placed, for example, under a kitchen sink, but which still provides relatively high throughput. In addition, the housing design is particularly adapted to provide internal flow of impure water around the membrane elements to insure that impurities are swept away from the surface efficiently. The housing also includes a safety system to prevent the possibility of back pressure from a pressurized purified water tank from damaging the membrane in the event of a failure of line pressure of the impure water source. Since membrane elements must be replaced after certain periods of use, the cartridge of the invention is also designed for easy installation and replacement.
Accordingly, it is an object of the invention to provide a reverse osmosis membrane element which is easy to manufacture, has a relatively high throughput, and is subject to a relatively low failure rate. It is another object of the invention to provide a membrane element having very carefully selected dimensions to permit maximum throughput while minimizing the potential for fatal flaws in the membrane surface. It is still another object of the invention to provide a mounting system for elements of the invention which is easily fabricated, and which permits easy installation and replacement of elements. These and other objects of the invention are recognized by the invention, a preferred embodiment of which is discussed herein.